1. Field of the Invention
The present invention relates to a focal plane shutter for cameras having one or two shutter blades.
2. Description of the Related Art
There is a focal plane shutter for cameras of a type having only one shutter blade including a plurality of arms fixed at one end to a base plate, and at least one blade rotatably supported by these arms. Such a focal plane shutter is employed only in digital cameras. A focal plane shutter having two of such shutter blade configured as described above is known since a long time ago, and this type of focal plane shutter is used both in digital cameras and silver salt film cameras. Such focal plane shutters are configured in such a manner that the respective shutter blades connect one arm to a drive pin of a drive member, so that an exposure aperture is opened and closed by reciprocal rotation of the drive member.
In the case of the focal plane shutter in the related art, the drive member is generally rotated in one direction by a set member interlocked with a motor, and in the opposite direction by an urging force of a spring. However, recently, a structure adapted to rotate the drive member reciprocally only by the motor attracts attention and an example in which the drive member is integrated with a rotor of the motor is disclosed in Japanese Patent Laid open Publication No. 2002-287210.
In the case of the focal plane shutter disclosed in Japanese Patent Laid open Publication No. 2002-287210, a stop position of the rotor is maintained by a detent torque obtained by a magnetic force of a permanent magnet of the rotor. However, when an impact or vibrations are applied to the camera, the shutter blade cannot be kept stand-still only by the detent torque. Therefore, there is a problem such that when the shutter blade is moved from the stop position, a start position at the time of photographing changes, and hence an adequate photographing cannot be achieved.
As publicly known, in the case of the focal plane shutter of this type, the drive member does not stop at the end of rotation thereof by the rotor of the motor being stopped, but is stopped by abutting against a stopper. At this time, when the drive member is bounded and is rotated temporarily in the opposite direction, also the shutter blade is operated temporarily in the opposite direction, and hence part of the exposure aperture is opened and closed, which may cause an inconvenient result such as uneven exposure.
In view of such circumstances, referring to the focal plane shutter disclosed in Japanese Patent Laid open Publication No. 2002-287210, a force of the drive member which comes into abutment with the stopper and bounds acts so as to rotate the rotor in the opposite direction from the current direction of rotation. Therefore, when the drive member is rotated at high velocities (that is, when the shutter blade is traveled at high velocities), the bound of the drive member as described above cannot be restrained preferably only by the detent torque which can be obtained by the magnetic force of the permanent magnet of the rotor, and hence the shutter blade may be operated temporarily in the opposite direction.
Therefore, in the case of the focal plane shutter as disclosed in Japanese Patent Laid open Publication No. 2002-287210, a rubber-made shock absorbing member is mounted at an edge of an elongated hole which serves as a stopper for the drive member, so that the bound of the drive member can be restrained as much as possible. However, as publicly known, when the drive member is rotated at high velocities, it is difficult to restrain the bound only with such shock absorbing member, it is additionally necessary to mount shock absorbing members of various shapes or to mount a brake member at different positions on a shutter base plate.